1. Field of the Invention
This invention relates to leuco dyes used for image formation, and which dyes are capable of yielding colored images having near infrared absorbance in the range of 600 to 1,000 nm with high stability.
2. Description of the Background
Recording materials using leuco dyes are conventionally known and used in practice, for example, as pressure-sensitive recording sheets and thermosensitive recording sheets. According to such use, a triphenylmethane leuco dye, a fluoran leuco dye, a phenothiazine leuco dye or an auramine leuco dye may be selected because these dyes have their own color tone.
Conventionally, such dyes have been developed, with an emphasis on improving the color tone, that is, on the absorption in the visible spectrum. Up to the present time, no dyes have been developed which can absorb in the near infrared region, i.e., a wave length of 700 to 1,000 nm, to a sufficient extent.
As the semiconductor laser has become prevalent, a tendency to read recorded images such as a bar code by use of the semiconductor laser has grown, and a demand for a thermosensitive recording sheet and a pressure-sensitive recording sheet which can absorb light in the near infrared region has also increased.
A variety of such leuco dyes capable of absorbing light in the near infrared region has been proposed recently, for example, monovinyl and divinyl phthalide compounds described in Japanese Laid-Open Patent Applications 51-121035, 51-121037, 51-121038, 57-167979 and 58-157779, and fluorene compounds in Japanese Laid-Open Patent Applications 59-199757 and 60-226871. However, the above leuco dyes have the shortcoming that their absorption intensity in the near infrared region is insufficient. In addition to this shortcoming, they have the shortcoming that the image formation stability is poor, which causes discoloration of colored images, and, moreover, the production cost is high.
Another near-infrared absorbing leuco dye is proposed in Japanese Laid-Open Patent Application 60-231766, which exhibits absorption in the near infrared region, but has the shortcoming that the preservability is poor, in particular, its heat resistance is poor.
Therefore, a need continues to exist for a leuco dye which can satisfy all of the above-mentioned requirements.